Due to improvements in GCC, the following combination is not supported:

  - GCC version 5 and above
  - x86
  - PIC (position-independent code)
  - low-level C grades using non-local gotos, i.e. asm_fast.*

The configure script will not select asm_fast grades when the above combination
is detected. If you try to use that combination by selecting an asm_fast grade
manually, you will encounter compilation errors.

We recommend using high-level C grades (hlc.*) on x86. If you require a
low-level C grade, e.g. for debugging, then you can use a reg.* grade.

If you must use an asm_fast grade, you will need to tell the C compiler not to
generate position-independent code (which may be the default), *and* disable
building of shared libraries. Alternatively, you could downgrade to gcc 4.9.

Note that Windows is unaffected by this issue as Windows does not use PIC,
and we do not yet support shared libraries (DLLs) on Windows anyway.

Also note x86-64 is not affected by this issue, only x86.

<https://bugs.mercurylang.org/view.php?id=453>
